How to create list inside for loop?

Question

How can I create lists using for loop to get list_1, list_2, list_3, etc.? (Python)

I tried this:

for i in range(5):
i += 1
list_(i) = []

But not working.

  File test.py, line 3
runner_(i) = []
            ^

SyntaxError: cannot assign to function call

What happens when you run your code? What do you expect to happen? As it is in the question you will just get an Indentation Error and it will stop. — JeffUK, Nov 15 '20 at 16:01
Does this answer your question? [Dynamic variable name in python](https://stackoverflow.com/questions/2564140/dynamic-variable-name-in-python) — JeffUK, Nov 15 '20 at 16:03
did you mean to create a dictionary with `{'list_1': [], 'list_2' = []}`? — hiro protagonist, Nov 15 '20 at 16:04

score 2 · Answer 1 · answered Nov 15 '20 at 16:06

2

You may create list of lists:

main_list = []
for i in range(5):
    sub_list= []
    main_list.append(sub_list)
print(main_list)
print(main_list[0])

answered Nov 15 '20 at 16:06

Savostyanov Konstantin

345
1
8

score 1 · Answer 2 · edited Nov 15 '20 at 18:37

1

dic={}
for i in range(5):
    dic[f"list{i}"]= []

print(dic)

edited Nov 15 '20 at 18:37

Eric Aya

69,473
35
181
253

answered Nov 15 '20 at 16:02

Shad0w

152
2
13

this is a example – Shad0w Nov 15 '20 at 16:02
In my opinion this is the best way to do what you really want to do – Shad0w Nov 15 '20 at 16:04
it makes a dictionary with 5 lists and you can access the dictionary like accessing a list – Shad0w Nov 15 '20 at 16:20

bbnumber2 · Answer 3 · 2020-11-15T18:12:35.180

Creating multiple variables is not considered Pythonic. If you want to create a series of variables, a good option is to use a list, like so:

list_of_lists = []
for i in range(5)
    list_of_lists.append([])

When you want to access a specific list:

list_of_lists[3]
-> []

However, if you would like to maintain the variable names, you could consider using a dictionary, perhaps like so:

dict_of_lists = {}
for i in range(5):
    dict_of_lists[f'list_{i}'] = []

Accessing:

dict_of_lists['list_1']
-> []

TS_ · Answer 4 · 2020-11-15T16:19:39.680

1

I would strongly recommend you to not do this at all. Creating dynamic variables is rarely a good idea and it might affects performance. You can always use dictionary instead as it would be more appropriate:

lists = {}
lists["list_" + str(i)] = []
lists["list_" + str(i)].append(somevalue)

Look here for some more explanation: Creating a list based on value stored in variable in python

edited Nov 15 '20 at 16:19

answered Nov 15 '20 at 16:06

TS_

319
1
5

score 1 · Answer 5 · answered Nov 15 '20 at 16:30

1

Ideally you should use list or dictionary. But, use the below code for the requirement.

     for i in range(5):
         exec("list_%d = []" % (i+1))

answered Nov 15 '20 at 16:30

vivek kumar

304
1
8

You're correct that the OP most likely needs to use a list or a dictionary. There are almost never a situation where `locals`, `globals` or `exec` is the solution. Using `exec` like this is more of a hack that will lead to confusion and potentially many future problems. – Ted Klein Bergman Nov 15 '20 at 16:51

score 0 · Answer 6 · answered Nov 15 '20 at 16:06

0

Try with globals():

for i in range(5):
  i += 1
  globals()['list_'+str(i)] = []

Most awesome, cool and pythonic way is BTW to use a dictionary man ;-)

list_dicts = {}
for i in range(5):
  i += 1
  list_dicts['list_'+str(i)] = []

And retrive/edit like:

list_dicts['list_2']

Tired of the sucking for loop? Time for dict comprehension:

list_dicts = {'list_'+str(i): [] for i in range(5)}

answered Nov 15 '20 at 16:06

Wasif

14,755
3
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34

There are almost never a situation where `locals`, `globals` or `exec` is the solution. So while `globals` works in this case, it's definitely not the correct approach. It's more of a hack that will lead to confusion and future problems. Your other suggestions are much better. – Ted Klein Bergman Nov 15 '20 at 16:53

Kaszanas · Accepted Answer · 2020-11-16T01:37:28.640

0

For this task I would choose a different data structure dict()

This will let You map the keys to respective lists such as:

my_dictionary = {}
for i in range(5):
    my_dictionary[f"list_{i}"] = [] # This might be also my_dictionary["list_"+str(i)]

Please note that I have used empty list as key -> value mapping.

This in turn will produce a following dictionary:

{"list_0": [], "list_1": [], "list_2": [], "list_3":[], "list_4":[]}

edited Nov 16 '20 at 01:37

answered Nov 15 '20 at 16:06

Kaszanas

446
4
18

or as a dict-comprehension: `{f"list_{i}": [] for i in range(5)}` . – hiro protagonist Nov 15 '20 at 16:10
Exactly as mentioned by @hiroprotagonist which would be a little less readable for some coming from different languages. – Kaszanas Nov 15 '20 at 16:10
1

`"list_"+i` will throw an error. You must convert the integer to a string first. – Ted Klein Bergman Nov 15 '20 at 16:54
Ah yes thank You @TedKleinBergman that is true, I am far more used to f-strings. – Kaszanas Nov 16 '20 at 01:38

score 0 · Answer 8 · answered Nov 15 '20 at 16:08

0

I think you are searching for the locals() function, this function holds all the local variables. you could then write something like this.

for i in range(5):
   locals()["list_"+str(i)] = []

answered Nov 15 '20 at 16:08

Glenn

27
4

While it works, it's definitely not the correct approach. It's more of a hack that will lead to confusion. The OP most likely needs to use a list or a dictionary. There are almost never a situation where `locals`, `globals` or `exec` is the solution. – Ted Klein Bergman Nov 15 '20 at 16:49

score 0 · Answer 9 · answered Nov 15 '20 at 16:41

Hope this is your answer

for x in range(5):
    x += 1
    list_name = "list_" + str(x)
    list_item = []
    print("Printing {0} = ".format(list_name), list_item)

Here is output:

Printing list_1 =  []
Printing list_2 =  []
Printing list_3 =  []
Printing list_4 =  []
Printing list_5 =  []

How to create list inside for loop?

9 Answers9